Devices exist which include a measuring plate which has measurement scales on first portions of the measuring plate which are projected onto and through the measurement scales on second portions of the measuring plate, or a pattern on a fixed screen is projected onto and through the second portions of the measurement scales on the measuring plate, and then onto a photo-detector allotted to each measurement scale to create electric measurement signals varying according to the movements of the measuring plate. Electric circuits are provided to interpret the measurement signals.
With measuring devices such as the one described above, it is often desirable to obtain a reference indication when the measuring plate occupies a particular position. According to German Pat. No. 1,814,785, it is known to install a special reference scale on a measuring plate which is scanned with a separate photo-detector to obtain a reference signal which is separate from the electric measurement signals.
Therefore, in order to obtain a reference signal in presently available systems, there is a need for both a separate photo-detector and a special reference scale on the measuring plate. Since a separate photo-detector and associated circuit is used shifts can occur in the reference signal due to temperature changes and aging of parts. This invention is chiefly characterized by the fact that such separate photo-detector and scale is unnecessary. Thus the device of this invention is simpler and cheaper than the systems at present available, while angle or length measuring devices without reference possibilities can easily be supplemented to include such adjustments. In this way an existing measurement scale can be easily supplemented with the help of modern masking methods so as to have qualities similar to those of the measurement scales to be described later, and that the existing electronic circuits can easily be connected to additional circuits for reference signal notations. In addition, displacements between reference signals and measurement signals caused, for example, by temperature changes or worn-out parts can be largely eliminated by means of the present invention.